Hydraulic system

ABSTRACT

This invention relates to a hydraulic system characterized by a combination pressure and utilization means unit assembly, and a dual low pressure hydraulic piping system between the pressure unit and a hydraulic fluid reservoir, thereby providing continuous circulation of hydraulic fluid during operation.

United States Patent Hoffman I [54] HYDRAULIC SYSTEM [72] Inventor:Bernard L. Hoffman, Trenton, NJ.

[73] Assignee: Fredrick A. Krause Associates, Inc.,

Frenchtown, NJ.

[22] Filed: April I, 1970 [21] Appl.No.: 24,567

511 mu ..B2ld 29/14 [58] Fieldol'Search ..72/3s0,3s1,4s3

[56] References Cited UNITED STATES PATENTS 1,696,140 12/1928 Ferris..72/3s1 1,696,141 12/1928 Ferris..... ..72/351 2,261,060 10/1941Giesler ..72/347 1 July 25,1972

Burk .Q. ..72/s7 Williamson 1 ..72/35l Williamson .....72/351 FOREIGNPATENTS OR APPLICATIONS 652,601 11/1937 Germany 1 ..72/351 1,040,48810/1958 Germany "72/351 Primary Examiner-Richard J. HerbstAttorney-Ward. McElhunnon, Brooks & Fitzpatrick l 5 ABSTRACT Thisinvention relates to a hydraulic system characterized by a combinationpressure and utilization means unit assembly, and a dual low pressurehydraulic piping system between the pressure unit and a hydraulic fluidreservoir, thereby providing continuous circulation of hydraulic fluidduring operation.

4 Claims, 4 Drawing Figures Patented July 25, 1972 3,678,726

2 Sheets-Sheet l HYDRAULIC SYSTEM This invention relates to hydraulicsystems and more particularly it concerns a novel hydraulic pressureunit suitable for use with multiple die punch presses.

Modern presses which draw or otherwise form various intricate shapesfrom sheet metal utilize complex multiple dies which are hydraulicallyactuated. These dies are made up of a plurality of die elements, each ofwhich is connected to actuating pistons of various size. The actuatingpistons in turn are supplied with hydraulic fluid at a common pressureso that when the die heads of the press are opened, or displaced fromone another, the hydraulic pressure maintains the pistons and theircorresponding die elements in fully extended position. As the die headsclose upon each other with the sheet metal workpiece therebetween, thedie elements become retracted in a certain predetermined sequence,depending, at least in part, upon the areas of the pistons actuatingthem. As the die heads are thereafter opened, the die elements begin toextend again in reverse sequence. It is this specially controlledsequence of retraction and extension of the various die elements whichpermits various intricate shapes to be produced in a single operation ofthe press.

The above described die arrangement presents certain requirements of thehydraulic system which supplies fluid under pressure to the actuatingpistons. This system, for example, must be capable of supplying asufficient quantity of hydraulic fluid and at a sufficient pressure tocause full extension of all actuating pistons and their associated dieelements when the die heads are opened. This pressure must not be toohigh, or otherwise, when the press is opened, the die elements will movetoward their extended positions with such suddenness that the formedworkpiece will be torn violently out of the dies and destroyed orseverely damaged. On the other hand, the hydraulic system must becapable of absorbing all of the displaced fluid during the closing ofthe die heads; and furthermore, it must closely maintain the pressure ofthe fluid in the system at a very high value during this time so thatthe workpiece can be properly formed by the extended die elements.

In addition to the above, a further problem arises from the fact thatthe rapid operation of the press produces very large changes in volumerates of flow of the hydraulic fluid in the system. Such rapid changesin flow rate produce very serious hydrodynamic effects which adverselyaffect valve operation, and of course, seriously affect die operation.These adverse hydrodynamic effects include fluid foaming and cavitation,pressure reflections and surges, slow valve response time and unstablevalve operation.

According to this invention, there is provided a new and improvedhydraulic system which employs a combination pressure and utilizationmeans, which may be a die cushion unit, for example. The system employsa low pressure hydraulic supply line and a separate hydraulic returnline between a hydraulic reservoir and the pressure and die cushionunit. This allows the hydraulic fluid to circulate between the reservoirand the pressure unit, resulting in the system operating at a lowtemperature and without foaming. Hence, the system can be operated atincreased strokes perv minute as compared to prior art systems.

In view of the foregoing, this invention contemplates the provision ofa,novel hydraulic system characterized by a pressure unit assembly,utilization means connected directly to the pressure unit assembly, anda hydraulic fluid reservoir assembly. A first hydraulic line serves tocarry fluid from the reservoir to the pressure unit assembly and asecond hydraulic line serves to return the fluid from the pressure unitassembly to the reservoir. The pressure unit assembly has a chamber influid flow communication with the first hydraulic line and with theutilization means. A check valve is mounted in the first hydraulic lineadjacent the pressure unit assembly for permitting fluid flow from thereservoir into the chamber and preventing flow in the oppositedirection. An internal passage interconnects the chamber with the secondhydraulic line and a valve is mounted in this passage. The valve isbiased to closure of the passage and means are provided for opening thevalve responsive to variations in static fluid pressure in the chamber,thereby to allow the fluid to return to the reservoir.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described more fullyhereinafter. Those skilled in the art will appreciate that theconception on which this disclosure is based may readily be utilized asthe basis for the designing of other structures for carrying out theseveral purposes of the invention. It is important, therefore, that thisdisclosure be regarded as including such equivalent constructions as donot depart from the spirit and scope of theinvention.

One embodiment of the invention has been chosen for purposes ofillustration and description, and is shown in the accompanying drawings,forming a part of the specification, wherein:

FIG. I is a side elevation, partially in section, showing a hydraulicsystem, adopted for operating a press, constructed in accordance withthe concept of this invention;

FIG. 2 is a medial, vertical, sectional view of a combination pressureunit and die cushion unit assembly;

FIG. 3 is a medial, vertical, sectional view of the pressure unitassembly of FIG. 2, but shown in another position thereof; and

FIG. 4 is an enlarged side elevation view of the pressure unit assemblyof FIG. 2.

In the embodiment of the invention illustrated, as best seen in FIG. 1,the hydraulic system comprises a press indicated generally at 10, havinga die cushion assembly indicated generally at I l, a pressure unitassembly indicated generally at 12, and an oil reservoir assemblyindicated generally at 14. According to the present invention hydraulicfluid or oil is recirculated through the entire system with each stroke,and hence, it operates cooler andwith less foaming than prior artnoncirculating systems. A hydraulic circuit extends from the oilreservoir 14 through low pressure hydraulic line I6 to the pressure unitassembly 12, and thence internally to the die cushion assembly 11.Further, the oil circuit extends internally back from the die cushionassembly Ilto the pressure unit assembly 12 and then to the oilreservoir 14 via a low pressure hydraulic return line 18, provided forthe purpose. The internal circuit will be described more fullyhereinafter. The system includes cooling means which comprise a watercircuit wherein water enters die cushion l l, as at 20, and iscirculated around each cylinder, line 22 serving to carry the water fromone cylinder to the next. Then, the water circuit extends internallyfrom the die cushion to the pressure unit assembly 12, and thereafterinto the reservoir 14, as at 24, via line 26. Finally, the water entersa cooling coil 28 in the reservoir to cool the oil therein, and then itis discharged, as at 30. A temperature sensing unit 32 is mounted on thepressure unit assembly l2, and it is set to operate at a preselectedtemperature to open a main electrical circuit (not shown) to deactivatethe equipment if the oil temperature exceeds the preselected temperatureso as to avoid damage to the equipment as well as preventing thestamping of defective parts in the die. As an example of one embodimentof the invention, the input water temperature is 55 F. and the outputwater temperature is 60 F., thereby maintaining an oil temperature of 76F. The temperature sensing unit is set to open the main electricalcircuit at I20 F. Such a unit has been operated at 350 strokes perminute.

Still referring to FIG. 1, the reservoir 14 is mounted on a pedestal 34.This reservoir comprises a body 36 having a bottom 38 with a settlingring 40, and a cap 42. A glass sight gauge 44 is mounted on the outsideof the reservoir to provide visual indication of the liquid leveltherein. Hydraulic fluid or oil is supplied to the reservoir via a fluidintake nozzle, not

shown, and for normal operation the reservoir is maintained at abouttwocthirds capacity. The fluid in the reservoir is maintained at apositive pressure (about I to 135 p.s.i.) during normal operation; andfor this purpose an air inlet 46 is employed for supplying air pressureon the upper surface of the oil in the reservoir.

As best seen in FIGS. 2 and 4, the pressure unit assembly 12 is formedwith a body 48 having an annular chamber 50 which is connected to thehydraulic line 16, FIG. 2, and a check valve 52 is interposed to permitfluid flow from the reservoir 14 into the chamber 50, but not in thereverse direction. A bias spring 54 in the check valve is quite lightand whenever the pressure in the chamber 50 drops below the reservoirpressure, the unbalance will overcome the spring force and allow fluidto flow out of the reservoir 14 through the hydraulic line 16 into thechamber 50. An excess of pressure in the chamber 50, on the other hand,will only serve to set the check valve more tightly so that no fluid canpass back into the reservoir 14, through the hydraulic line 16. Thechamber 50 is of annular configuration and is connected to a utilizationmeans, such as the die cushion assembly 11, as will be described morefully hereinafter.

The annular chamber 50 opens directly into a lower chamber 56 whichopens directly into a lower connecting passageway 58 which leads to thehydraulic return line I8, FIG. 2. A control valve, indicated generallyat 60, is mounted on the pressure body 48 to control the opening of thepassageway 58, and thereby control the flow of oil to the return line18. i

The control valve 60, FIGS. 2 and 3, has a control valve spool 62 whichis cylindrical in shape and includes two large diameter end portions 64and 66, and a smaller diameter intermediate portion 68. The end portions64 and 66 are hollowed out, to provide a pressure responsive chamber 70and a spring seating chamber 72. These two chambers are maintained influid communication by means of a central passageway 74 extendingaxially of the spool 62. The fluid flow velocity through the passagewayis restricted to a predetermined value by means of a metering orificeinsert 76 press fitted into the passageway.

The first larger diameter end portion 64 of the valve spool 62 extendsinto an opening 78, and the second larger diameter end portion 64extends into an opening 80 in the body 48. The two openings 78 and 80are aligned with and of the same 'diameter as the lower connectingpassageway 58. The control valve spool 62 is thus reciprocally movablebetween a passageway closing position as seen in FIG. 2, wherein thesecond larger diameter portion 66 fully covers the lower connectingpassageway 58, and a passageway opening position as shown in FIG. 3,wherein oil can flow from the lower chamber 56 through the lowerconnecting passageway 58 to the hydraulic return line [8. A valve spoolbias spring 82 fits into the spring seating chamber 72 of the valvespool 62, and urges the valve spool toward the left as viewed in FIG. 2,to its valve closing position.

A pressure end cap 84 and an adjustment end cap 86 are bolted to theouter walls of the body 48, and they serve to cover the'valve spoolopenings 78 and 80. The pressure end cap 84, as shown in FIG. 2, isprovided with an internal passageway 88 which, as shown, communicatesbetween the lower chamber 56 and the pressure responsive chamber 70 ofthe control valve spool 62. Pressure changes in the chamber 56 arecommunicated via the passageway 88 and act upon the left end of thespool 62, as viewed in FIG. 2, in opposition to the force of the spoolbias spring 82 to move the spool to the right as viewed in FIG. 3 andopen the lower connecting passageway 58 between the chamber 56 and thehydraulic retum'line 18.

The adjustment end cap 86 is provided with an adjustment needle valve 90which communicates via an opening 92 with the spring seating chamber 72of the valve spool 62 and through the central passageway 70 and themetering orifice insert 76, to the pressure responsive chamber 70. Theneedle valve is biased against a needle valve seat 94 by an adjustmentvalve spring 96, the stress on which is adjustably controlled by anadjustment screw 98 threaded into the end cap 86. A lock nut 100 isprovided on the adjustment screw for securing it at any given setting.As shown in FIG. 2, the region 102 beyond the adjustment valve 90 is incommunication, via a feedback passageway 104 with the annular chamber50. There is also provided safety valve means, not shown, for thechamber 50.

As best seen in FIG. 2, the annular chamber 50 is connected to the diecushion assembly 11. This die cushion assembly includes a die cushionbody 104 fixedly mounted on the pressure unit body, as at 106, and a diecushion rod 108 passes therebetween. A cover plate 109 is mounted on theupper end of the body 104. Four piston sleeves 110 (two being shown) aremounted in symmetrically spaced openings 2 in the body 104 for receivingpiston assemblies 114. The piston assemblies I14 engage die elements116. In order to achieve complex working of the metal workpiece in asingle pressure operation, certain of the die elements are movable ondie shoes 118, FIG. I, and are urged toward protracted or extendedpositions on the shoes by actuating the piston assemblies 114 which maybe of various cross sectional areas. These actuated pistons are in fluidcommunication with the chamber 50in the pressure unit body 48. As thepress closes and the die shoes are moved toward each other with greaterand greater force, the die actuating piston assemblies and theirassociated die elements are forced back into their respective die shoesin predetennined sequence according to the cross sectional area of thepistons.

The press unit 12 operates to receive and transmit via hydraulic returnlines 18 to the reservoir 14, the fluid displaced by the retractingpistons during press closure while maintaining the high resistancepressure (in the neighborhood of 4,500 p.s.i.) required for proper metalworking, and when the press opens the pressure unit 12 acts to supplyoil from the reservoir via hydraulic line 16 to the piston assembliesI14 for repositioning the die elements and ejecting the previouslyfinished workpiece. It will be appreciated that only the pressure unitassembly between the chamber 50 and the spool valve 62 are subjected tothe very high pressures, and hence the hydraulic lines 16 and 18 andtheir respective fittings may be designed for low pressure operation.

As pointed out hereinbefore, the system is additionally provided withcooling means. The cooling water which enters the die cushion assemblyat 20 circulates around the piston sleeves 110, as at 120, FIG. 2. Thiscooling water also passes through various internal passageways 122within the body 48 for cooling purposes.

The system operates in the following manner:

When the press opens, and the die shoes 118, FIG. I, are moved apart,there is no retracting force on the actuating pistons 114. Consequently,the pressure in the annular chamber 50, lower chamber 56, passageway 58and return line 18 falls off toward atmospheric pressure. Similarly, thepressure in the pressure responsive chamber 70 of the control valvespool 62 also decreases because this chamber is in fluid flowcommunication with the lower chamber 56 via the internal pressurepassageway 88. Consequently, the valve spool bias spring 82, receivingno opposition, moves the valve spool 62 to the left, as viewed in FIG.2, so that the larger diameter end portion 66 of the spool fully coversthe lower connecting passageway 58. However, when this decreasedpressure falls below the pressure in the reservoir 14, a force unbalanceexists across the check valve 52 causing it to unseat so that fluid isforced at about 100-125 p.s.i. from the reservoir 14 via hydraulic line16 to the annular chamber 50 and thence to the piston assemblies 114where it forces the actuating pistons to their extended positions.

When the press is closed and the die elements are brought togetheragainst a workpiece, they force back against the actuating pistonassemblies 114 causing a pressure increase in the annular chamber 50.This increased pressure acts to close the check valve 52. However, italso communicates through the lower chamber 56 and internal passageway88 to the pres sure responsive chamber 70 in the control valve spool 62and acts upon that end of the spool in opposition to the valve spoolbias spring $2. This causes the valve spool to move to the right, asviewed in FIG. 2, which, in turn, brings about communication between thechamber 56 and the hydraulic return line 18 to the reservoir 14.Accordingly, the fluid which is displaced by the retracting pistonassemblies during closure of the press is accommodated through thecontrolled opening of the lower connecting passageway 58 by movement ofthe valve spool 62. It will be appreciated that even though theactuating pistons operate in a reciprocating manner, the hydraulic fluidpasses in stepwise movement through the entire circulatory path so thatcool fluid is continuously brought into the piston area.

It is important in many applications, particularly in the hydraulicpress situation described herein, to accommodate the rapid volumetricdisplacements caused by the retracting piston and yet to maintain thepressure in the system at close to 4,500 psi. Also, because variousdifferent pistons begin to retract at different points ,during pressclosure, the system must be capable of accommodating large surges inboth pressure and volume, without the loss of control and valve flutterwhich often plague conventional valving systems under such conditions.

in the present system, the two larger diameter end portions 64 and 66 ofthe control valve spool 62 are of equal diameter. Thus, the pressuresurges which occur in the chambers 50 and 56 are balanced out at the twoend portions of the spool, All control of spool movement occurs as aresult of pressure communication through the internal pressurepassageway 88 to the end of the spool. Accordingly, the spool does notsuffer from the hydrodynamic effect of the fluid rushing through it.Further, the various changes in direction the fluid must take betweenthe chamber 56 and the pressure responsive chamber 70 causes adissipation of surge effects so that the valve operates only in responseto actual pressure changes and not to surge effects.

As pressure increases at the left end of the valve spool 62, it isforced rightward against the valve spool bias spring 82. This forces allfluid in the spring seating chamber 72 out through the opening 92 andagainst the adjustment needle valve 90, causing it to open when thepressure reaches a sufficient value, i.e., the value corresponding to4,500 psi. in the chambers 50 and 56. Even while this pressure issustained, hydraulic fluid passes through the central passageway 70 andthe metering orifice insert 76 and into the spring seating chamber 72where it balances the pressure in the pressure responsive chamber 70.This permits the valve spool bias spring 82 to move the spool 62 backtowards the left until the flow through the lower connecting passageway58 is decreased to a point where the pressure in the system again buildsup. The valving arrangement is thus dynamically balanced and in fact iscapable of handling pressure and volume variations at rates appreaching80 cycles per second without causing piston bounce, oil foaming orentrapped air.

It will be appreciated that in certain installations two or more diecushions may be employed per die. In such installations only onereservoir is used but a plurality of oil and return lines are employedto avoid pressure reactions between cushions;

It will thus be seen that the present invention does provide an improvedhydraulic system which is superior in simplicity, compactness andefficiency as compared to prior art such systems.

Although a particular embodiment of the invention is herein disclosedfor purposes of explanation, various modifications thereof, after studyof this specification, will be apparent to those skilled in the art towhich the invention pertains,

What is claimed and desired to be secured by Letters Patent is:

l. A hydraulic system comprising a pressure unit assembly, a die cushionassembly connected directly to said pressure unit assembly, and aremotely disposed hydraulic fluid reservoir assembly, water coolingmeans for circulating cooling water through said die cushion assemblyand said pressure unit assembly and said hydraulic fluid reservoirassembly, a first hydraulic line for carrying fluid from said reservoirassembly to said pressure unit assembly and a second hydraulic line forreturning fluid from said pressure unit assembly to said reservoirassembly, said pressure unit assembly having a pressure unit body, saidbody having a chamber in fluid flow communication with said firsthydraulic line, a check valve mounted in said first hydraulic lineadjacent said body for permitting fluid flow from said reservoirassembly into said chamber and preventing fluid flow in the oppositedirection, said body having a lower connecting passagewayinterconnecting said chamber with said second hydraulic line forreceiving substantially all of the flow from said chamber, a controlvalve spool mounted for movement between a first position closing saidlower connecting passageway is open, fluid passage means for exposingone end of said control valve spool to the fluid pressure in saidchamber for controlling the movement of said control valve spool, saiddie cushion assembly having a die cushion body, a piston assemblymounted in an opening in said die cushion body, said piston assemblybeing in fluid flow communication with said chamber, and a die elementmounted for engagement with said piston assembly.

2. A hydraulic system according to claim 1 wherein said control valvespool has two oppositely disposed large diameter end portions and asmaller diameter intermediate portion, one

of said end portions having a pressure responsive chamber and the otherof said end portions having a spring seating chamber, a centralpassageway extending axially of said spool for connecting said pressureresponsive and spring seating chambers, said body having oppositelydisposed openings for receiving said end portions in slidable contact, aspring mounted in said spring seating chamber for urging said controlvalve spool in one direction wherein one of said end portions closessaid lower connecting passageway, an internal passageway communicatingbetween said chamber and said pressure responsive chamber forcommunicating pressure changes in said chamber to said pressureresponsive chamber to urge said control valve spool to move inopposition to said spring for opening said lower connecting passageway.

3. A hydraulic system according to claim 2 further compris ing anadjustable needle valve assembly mounted on said body adjacent saidspring seating chamber, said needle valve assembly having an openingconnecting to said spring seating chamber and a passageway connecting tosaid annular chamber, adjustable spring means for closing said needlevalve assembly to fluid flow when the pressure in said opening is belowa selected valve and for allowing fluid flow through the needle valveassembly when the pressure in said opening is above said value.

4. A hydraulic system comprising a pressure unit assembly, a die cushionassembly connected directly to said pressure unit assembly, and ahydraulic fluid reservoir assembly, water cooling means for circulatingcooling water through said die cushion assembly and said pressure unitassembly and said hydraulic fluid reservoir assembly, a first hydraulicline for carrying fluid from said reservoir assembly to said pressureunit assembly and a second hydraulic line for returning fluid from saidpressure unit assembly to said reservoir assembly, said pressure unitassembly having a pressure unit body, said body having an annularchamber in fluid flow communication with said first hydraulic line, acheck valve mounted in said first hydraulic line adjacent said body forpermitting fluid flow from said reservoir assembly into said chamber andpreventing fluid flow in the opposite direction, said body having alower chamber opening directly into said annular chamber, said bodyhaving a lower connecting passageway interconnecting said lower chamberwith said second hydraulic line, a control valve spool having twooppositely disposed large diameter end portions and a smaller diameterintermediate portion, one of said end portions having a pressureresponsive chamber and the other of said end portions having a springseating chamber, a central passageway extending axially of said spoolfor connecting said pressure responsive and spring seating chambers, ametering orifice disposed in said central passageway, said body havingoppositely disposed openings for receiving said end portions in slidablecontact, said openings being aligned with and of the same diameter assaid lower connecting passageway, a spring mounted in said springseating chamber for urging said control valve spool in one directionwherein one of said end portions closes said lower connectingpassageway, an internal passageway communicating between said lowerchamber and said pressure responsive chamber for communicating pressurechanges in said lower chamber to said pressure responsive chamber tourge 'said control valve spool to move in opposition to said spring foropening said lower connecting passageway, an adjustable needle valveassembly mounted on said body adjacent said spring seating chamber, saidneedle valve assembly having an opening connecting to said springseating chamber and a passageway connecting to said annular chamber,adjustable spring means for closing said needle valve assembly to fluidflow when the pressure in said opening is below a selected value and forallowing fluid flow through the needle valve assembly when the pressurein said opening is above said value; said die cushion assembly having adie cushion body, four symmetrically spaced sleeves mounted in openingsin said die cushion body, a piston assembly mounted in each of saidsleeves, said piston assemblies being in fluid flow communication withsaid annular chamber, and die elements mounted for engagement with saidpiston assemblies, respectively.

l l I l Po-ww UNITED; STATES PATENT OFFICE u) CERTIFICATEOF CORRECTION VPatent NC? 8 5 Dated. Ji 25'', 1972 V I Inven t0 r(s) I IBERNARID v I Itis eer t if ie d that; efror appears i n' the above-identified patentand that said Letters Patent are'hereby corrected as shdwn bel'owz Col.3, line 2, ""two ct'hj rde Should read.-two thirds--; Col. 6, line 20,after the wordf'passagew ay insert the words -and a seeofld pos itibnw'h erein said passageway-; Col. 6, line 52 seleted valve"Qsh-ou'ld read--selecte 5l Valuer".

Si g ned and sealed this 2nd day o f January 1972;m

(SEAL) Attest:

" EDWARD M.FLETCHER,JR.

Attesting Officer ROBERT GOTTSCHALK Commissioner of Patent

1. A hydraulic system comprising a pressure unit assembly, a die cushionassembly connected directly to said pressure unit assembly, and aremotely disposed hydraulic fluid reservoir assembly, water coolingmeans for circulating cooling water through said die cushion assemblyand said pressure unit assembly and said hydraulic fluid reservoirassembly, a first hydraulic line for carrying fluid from said reservoirassembly to said pressure unit assembly and a second hydraulic line forreturning fluid from said pressure unit assembly to said reservoirassembly, said pressure unit assembly having a pressure unit body, saidbody having a chamber in fluid flow communication with said firsthydraulic line, a check valve mounted in said first hydraulic lineadjacent said body for permitting fluid flow from said reservoirassembly into said chamber and preventing fluid flow in the oppositedirection, said body having a lower connecting passagewayinterconnecting said chamber with said second hydraulic line forreceiving substantially all of the flow from said chamber, a controlvalve spool mounted for movement between a first position closing saidlower connecting passageway is open, fluid passage means for exposingone end of said control valve spool to the fluid pressure in saidchamber for controlling the movement of said control valve spool, saiddie cushion assembly having a die cushion body, a piston assemblymounted in an opening in said die cushion body, said piston assemblybeing in fluid flow communication with said chamber, and a die elementmounted for engagement with said piston assembly.
 2. A hydraulic systemaccording to claim 1 wherein said control valve spool has two oppositelydisposed large diameter end portions and a smaller diameter intermediateportion, one of said end portions having a pressure responsive chamberand the other of said end portions having a spring seating chamber, acentral passageway extending axially of said spool for connecting saidpressure responsive and spring seating chambers, said boDy havingoppositely disposed openings for receiving said end portions in slidablecontact, a spring mounted in said spring seating chamber for urging saidcontrol valve spool in one direction wherein one of said end portionscloses said lower connecting passageway, an internal passagewaycommunicating between said chamber and said pressure responsive chamberfor communicating pressure changes in said chamber to said pressureresponsive chamber to urge said control valve spool to move inopposition to said spring for opening said lower connecting passageway.3. A hydraulic system according to claim 2 further comprising anadjustable needle valve assembly mounted on said body adjacent saidspring seating chamber, said needle valve assembly having an openingconnecting to said spring seating chamber and a passageway connecting tosaid annular chamber, adjustable spring means for closing said needlevalve assembly to fluid flow when the pressure in said opening is belowa selected valve and for allowing fluid flow through the needle valveassembly when the pressure in said opening is above said value.
 4. Ahydraulic system comprising a pressure unit assembly, a die cushionassembly connected directly to said pressure unit assembly, and ahydraulic fluid reservoir assembly, water cooling means for circulatingcooling water through said die cushion assembly and said pressure unitassembly and said hydraulic fluid reservoir assembly, a first hydraulicline for carrying fluid from said reservoir assembly to said pressureunit assembly and a second hydraulic line for returning fluid from saidpressure unit assembly to said reservoir assembly, said pressure unitassembly having a pressure unit body, said body having an annularchamber in fluid flow communication with said first hydraulic line, acheck valve mounted in said first hydraulic line adjacent said body forpermitting fluid flow from said reservoir assembly into said chamber andpreventing fluid flow in the opposite direction, said body having alower chamber opening directly into said annular chamber, said bodyhaving a lower connecting passageway interconnecting said lower chamberwith said second hydraulic line, a control valve spool having twooppositely disposed large diameter end portions and a smaller diameterintermediate portion, one of said end portions having a pressureresponsive chamber and the other of said end portions having a springseating chamber, a central passageway extending axially of said spoolfor connecting said pressure responsive and spring seating chambers, ametering orifice disposed in said central passageway, said body havingoppositely disposed openings for receiving said end portions in slidablecontact, said openings being aligned with and of the same diameter assaid lower connecting passageway, a spring mounted in said springseating chamber for urging said control valve spool in one directionwherein one of said end portions closes said lower connectingpassageway, an internal passageway communicating between said lowerchamber and said pressure responsive chamber for communicating pressurechanges in said lower chamber to said pressure responsive chamber tourge said control valve spool to move in opposition to said spring foropening said lower connecting passageway, an adjustable needle valveassembly mounted on said body adjacent said spring seating chamber, saidneedle valve assembly having an opening connecting to said springseating chamber and a passageway connecting to said annular chamber,adjustable spring means for closing said needle valve assembly to fluidflow when the pressure in said opening is below a selected value and forallowing fluid flow through the needle valve assembly when the pressurein said opening is above said value; said die cushion assembly having adie cushion body, four symmetrically spaced sleeves mounted in openingsin said die cushion body, a piston assembly mounted in each of saidsleeves, said piston assemblies being in fluid flow communicaTion withsaid annular chamber, and die elements mounted for engagement with saidpiston assemblies, respectively.